1. Field of the Invention
The present invention relates to a method of bonding chip parts whereby a chip part having solder bumps on the underside is bonded to a base material and an apparatus of the same, particularly to the method of bonding chip parts suitable for use in replacing bare chips (flip chips) on an MCM (Multi Chip Module) board, etc. where bare chips are mounted, and an apparatus for the same.
2. Description of the Related Art
In recent years, the MCM (Multi Chip Module) has been used as a CPU in small-sized personal equipment and the like. On a board of the MCM, plural bare chips (chip parts) of LSIs and the like are mounted. On the underside of a bare chip, multiple solder bumps are provided in advance, at which input and output terminals of the bare chip are electrically connected to patterns on the board (base material), and at the same time, the bare chip is bonded onto the board. In manufacturing an MCM, plural bare chips are disposed at specific positions on a board. Heating, so as to melt the solder bumps, bonds the bare chips on the board at the same time.
After manufacturing the MCM, when the inspection result confirms a defective bare chip, only the defective bare chip is removed to be replaced with a new one. With such a replacement of a bare chip, in order to bond a new bare chip on a board, the impulse heating system and the hot air heating system have been applied as a technique to melt solder bumps on the underside of the bare chip.
FIG. 7 shows an example of an apparatus in which the impulse heating system is applied. This apparatus shown in FIG. 7 comprises a preheating plate 10, reflow tool 11, driving mechanism 12, and impulse controller 13.
The preheating plate 10, on which a work [board (base material) 1 and bare chip (silicon wafer) 2] to be replaced is mounted to be preheated, is incorporated with a heating coil (not illustrated) etc.
The reflow tool (heating tool) 11 comes into contact with the upper surface of a bare chip 2 and heats solder bumps 3 on the lower surface of the bare chip 2.
The driving mechanism 12 drives the reflow tool 11 vertically, which is built up with a mechanism such as a ball screw and a motor combined. This driving mechanism 12 is designed to be controlled through a feedback control loop by a controller (not illustrated) in accordance with a load detected by a load cell (not illustrated).
The impulse controller 13 applies a voltage to the reflow tool 11 to run a specific impulse current (for example, 60 A), and heats the reflow tool 11 through the resistance thereof. The impulse controller 13 maintains the reflow tool 11 at a constant stabilized temperature in accordance with a temperature of the reflow tool 11 detected by a thermocouple not illustrated.
In the foregoing apparatus in which the impulse heating system is applied, the board (base material) 1 is mounted on the preheating plate 10, and the bare chip 2 provided with multiple solder bumps 3 on the underside is mounted at a specific position on the board 1.
Thereafter, while the preheating plate 10 preheats the board 1, bare chip 2, and solder bumps 3, the driving mechanism 12 shifts down the reflow tool 11 to bring the reflow tool 11 into contact softly with the upper surface of the bare chip 2, then detects the height of the bare chip 2, and shifts up the reflow tool 11 to a height in which the thermal expansion of the board (for example, a base material of molybdenum) 1, the reflow tool 11, and the like is taken into consideration, thus positioning the reflow tool 11.
And the impulse heating controlled by the impulse controller 13 is applied to the reflow tool 11 to heat up to a specific temperature, and the solder bumps 3 on the underside of the bare chip 2 are melted by the heat. At that moment, since the board 1 and the reflow tool 11 expand by the heat of the reflow tool 11, the space between the board 1 and the reflow tool 11 shrinks; consequently, the bare chip 2 is pressed toward the board 1 through the melted solder bumps 3 to be bonded to the board 1.
On the other hand, in the apparatus not illustrated in which the hot air heating system is applied, while a bare chip is disposed at a specific position on a board, blown hot air melts the solder bumps, thus bonding the bare chip onto the board through the solder bumps.
However, in the aforementioned impulse heating system in FIG. 7, the reflow tool 11 directly comes into contact with the bare chip 2 to apply a pressure. Therefore, the following problems will occur unless the reflow tool 11 is positioned at an optimum height with the thermal expansion taken into consideration.
When the reflow tool 11 is positioned at a height much lower than the optimum, an excessive pressure is applied to the bare chip 2, so that the solder bumps 3 can be crushed to make a short-circuit or the bare chip 2 can be crushed. On the contrary, when the reflow tool 11 is positioned at a height much higher than the optimum, a sufficient pressure is not applied to the bare chip 2. Therefore, it is not possible to sink the bare chip 2 into the solder bumps 3, and not possible to reliably bond the bare chip 2 to the board 1. Thus, the accuracy of the reflow tool 11 and the like and the positioning accuracy influence the quality of the work (the board 1 of the MCM, etc. where the bare chip 2 is replaced).
On the other hand, in the hot air heating system, since hot air blows out so as to surround the work to be bonded, the other parts can be influenced by the heat. In addition, it is impossible to press a bare chip toward a board by applying an appropriate load, and adjusting a sinkage of the bare chip 2 is impossible.